1. Field of the Invention
The present invention relates to high density memory devices based on phase change based memory materials, including chalcogenide based materials and other programmable resistive materials, and to methods for manufacturing such devices.
2. Description of Related Art
Phase change based materials, like chalcogenide based materials and similar materials, can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher electrical resistivity that the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile circuits, which can be read and written with random access.
The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or break down the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from the crystalline to the amorphous state. The magnitude of the current needed for reset current can be reduced by reducing the size of the phase change material element in the cell and/or the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element.
A specific issue arising in phase change memory structures is the heat sink effect of the electrodes in contact with the phase change material. Because the phase change occurs as a result of heating, the relatively high thermal conductivity of the electrodes will act to draw heat away from the phase change material, resulting in a need for higher current to induce the desired phase change.
It is therefore desirable to provide a structure addressing the electrode heat sink issues described above, resulting in memory cell structures having low reset currents, and methods for manufacturing such structures which meet the tight process variation specifications needed for large-scale memory devices.